This invention generally relates to mass spectroscopic analysis and, more particularly, an apparatus including a multi-reflecting time-of-flight mass spectrometer (MR-TOF MS) and a method of use.
Mass spectrometry is a well-recognized tool of analytical chemistry, used for identification and quantitative analysis of various compounds and their mixtures. Sensitivity and resolution of such analysis is an important concern for practical use. It has been well recognized that resolution of time-of-flight mass spectrometers (TOF MS) improves with flight path. Multi-reflecting time-of-flight mass spectrometers (MR-TOF MS) have been proposed to increase the flight path while keeping moderate physical length. The use of MR-TOF MS became possible after introduction of an electrostatic ion mirror with time-of-flight focusing properties. U.S. Pat. No. 4,072,862, Soviet Patent No. SU1681340, and Sov. J. Tech. Phys. 41 (1971) 1498, Mamyrin et. al. disclose the use of an ion mirror for improving time-of-flight focusing with respect to ion energy. The use of an ion mirror automatically causes a single folding of ion flight path.
H. Wollnik realized a potential of ion mirrors for implementing a multi-reflecting MR-TOF MS. UK Patent No. GB2080021 suggests reducing the full length of the instrument by folding the ion path between multiple gridless mirrors. Each mirror is made of coaxial electrodes. Two rows of such mirrors are either aligned in the same plane or located on two opposite parallel circles (see FIG. 1). Introduction of gridless ion mirrors with spatial ion focusing reduces ion losses and keeps the ion beam confined regardless of number of reflections (see U.S. Pat. No. 5,017,780 for more details). The gridless mirrors disclosed in UK Patent No. GB2080021 also provide “‘independence of ion flight time from the ion energy’.” Two types of MR-TOF MS are disclosed:
(A) “folded path” scheme, which is equivalent to combining N sequential reflecting TOF MS, and where the flight path is folded along a jigsaw trajectory (FIG. 1A); and
(B) “coaxial reflecting” scheme, which employs multiple ion reflections between two axially aligned ion mirrors using pulsed ion admission and release (FIG. 1B).
The “coaxial reflecting” scheme is also described by H. Wollnik et al. in Mass Spec. Rev., 1993, 12, p. 109 and is implemented in the work published in the Int. J. Mass Spectrom. Ion Proc. 227 (2003) 217. Resolution of 50,000 is achieved after 50 turns in a moderate size (30 cm) TOF MS. Gridless and spatially focusing ion mirrors preserve ions of interest (losses are below a factor of 2), although the mass range shrinks proportionally with a number of cycles.
MR-TOF mass spectrometers have also been designed with using sector fields instead of ion mirrors (Toyoda et al., J. Mass Spectrometry, 38 (2003), 1125; Satoh et al., J. Am. Soc. Mass Spectrom., 16 (2005), 1969). However, these mass analyzers, unlike those based on ion mirrors, provide for only first-order energy focusing of the flight time.
Soviet Patent No. SU1725289 by Nazarenko et al. (1989) introduces an advanced scheme of a folded path MR-TOF MS, using two-dimensional gridless mirrors. The MR-TOF MS comprises two identical mirrors, built of bars, parallel and symmetric with respect to the median plane between the mirrors and also to the plane of the folded ion path (see FIG. 2). Mirror geometry and potentials are arranged to focus the ion beam spatially across the plane of the folded ion path and to provide second-order time-of-flight focusing with respect to ion energy. The ions experience multiple reflections between planar mirrors, while slowly drifting towards the detector in a so-called shift direction (the Z axis in FIG. 2). The number of cycles and resolution are adjusted by varying an ion injection angle. The scheme allows the retention of full mass range while extending the flight path.
However, the planar mass spectrometer by Nazarenko provides no ion focusing in the shift direction, thus, essentially limiting the number of reflection cycles. Besides, the ion mirrors used in the prototype do not provide time-of-flight focusing with respect to spatial ion spread across the plane of the folded ion path so that use of diverging or wide beams would in fact ruin the time-of-flight resolution and make an extension of flight path pointless.
In U.S. Pat. No. 7,385,187, filed Dec. 20, 2005, entitled MULTI-REFLECTING TIME-OF-FLIGHT MASS SPECTROMETER AND METHOD OF USE, the planar scheme of multi-reflecting mass spectrometer is improved by:                a) introducing an ion mirror which provides spatial focusing in the vertical direction, high order spatial and energy focusing while staying isochronous to a high order of spatial and energy aberrations;        b) introducing a set of periodic lenses in the field-free region, where such a lens system retains ion packets along the main jigsaw ion path; and        c) introducing end deflectors, which allow further extension of the ion flight path by reverting the ion motion in the drift direction.        
Further improvements of planar multi-reflecting TOF MS were made in the following applications by the inventors: WO2006102430, WO2007044696, and WO2004008481.
These applications describe multiple pulsed ion sources including various schemes of ion accumulation and conversion of continuous ion beams into short ion packets. WO2006102430 suggests a curved isochronous interface for ion injection from external pulsed ion sources into the analyzer. The interface allows bypassing fringing fields of the analyzer and this way improves resolution of the instrument. The curved interface is compatible with trap ion sources and with the pulsed converter based on orthogonal ion acceleration.
WO2007044696 suggests a so-called double orthogonal injection of ions into the MR-TOF. Accounting that the MR-TOF analyzer is much more tolerant to vertical Y-spread of ion packets, a continuous ion beam is oriented nearly orthogonal to the plane of jigsaw ion trajectory in MR-TOF. The accelerator is slightly tilted and ion packets are steered after acceleration such that to mutually compensate for tilting and steering.
WO2004008481 applies an MR-TOF analyzer to various tandems of TOF MS. One scheme employs slow separation of parent ions in the first MR-TOF and rapid analysis of fragment ions in the second short TOF MS to accomplish so-called parallel MS-MS analysis for multiple parent ions within one shot of the pulsed ion source.
WO2005001878 is considered a prototype of the present invention, since it employs “folded path” MR-TOF MS with planar gridless mirrors, having spatial and time-of-flight focusing properties.
While implementing planar multi-reflecting mass spectrometers, the inventors discovered that the system of periodic lens commonly interferes with ion injection interface and pulsed ion sources. Also, the lens system sets the major limitation onto acceptance of the analyzer. The goal of the present invention is to improve sensitivity and resolution of multi-reflecting mass spectrometers as well as to improve convenience of their making.